Air spring stiffness controller

ABSTRACT

A method and apparatus for adaptively damping relative motion between the wheels and the frame of a heavy duty truck having a frame suspended on wheels by a suspension system. An air spring is disposed between the frame and at least one of the wheels having a primary reservoir for holding air and a piston adapted to act upon the air in the reservoir to compress the air and thereby provide support for the frame. An auxiliary reservoir holds air and can be placed in fluid communication with the primary reservoir to increase an effective volume of air upon which the piston acts. A control valve selectively places the auxiliary reservoir in communication with the primary reservoir based on vehicle operating parameters. The control valve can be actuated mechanically by forces acting on the wheels or by a controller that controls the valve based on wheel torque and road roughness.

TECHNICAL FIELD

[0001] The invention relates generally to suspension systems for overthe highway, heavy duty trucks and more particularly to a lockingsuspension system that varies the stiffness of suspension components inresponse to operating conditions.

BACKGROUND OF THE INVENTION

[0002] The frames of heavy duty trucks are typically suspended on torquereactive rear suspensions. Commonly, these suspensions provide a leaf orair spring to maintain a constant vehicle height. As drive train systemsare optimized to provide greater torque output, suspensions in generalhave been made stiffer to compensate for torque reaction and drivelinevibrations that are associated with the higher torques. While suchcompensation has diminished the effects of torque reaction and drivelinevibrations to provide a relatively comfortable ride during normaloperating conditions, the stiffer suspensions cannot absorb high impactforces such as those caused by rough roads and as a result do notprovide adequate cushioning against such events.

[0003] Several techniques have been used in the art to adaptivelycontrol the stiffness characteristics of vehicle suspension components,such as shock absorbers or air springs, in response to operatingcharacteristics. For example, U.S. Pat. No. 4,564,214 to Tokunaga et al.discloses a shock absorber having a an air chamber that serves as anintegral air spring. The air chamber is in fluid communication with anauxiliary reservoir to provide a relatively “soft” ride during normaloperating conditions. When the steering wheel is turned, the air chamberis disconnected from the auxiliary reservoir, which results in a smallerreservoir and stiffer ride characteristics, to provide a relatively“hard” ride during the steering event to enhance vehicle control. U.S.Pat. No. to 6,276,710 to Sutton discloses a system of air springs for avehicle tandem axle in which air springs on the same side of the vehicleare selectively placed in fluid communication with one another toprovide a relatively “soft” ride by virtue of effectively increasing thevolume of air in the reservoir over the volume of air in the reservoirof each air spring by itself. When the vehicle body begins to roll, theair springs are disconnected to provide better control until the bodyreturns to its normal orientation.

SUMMARY OF THE INVENTION

[0004] A heavy duty truck suspension system having air springs whosestiffness characteristics can be adjusted according to operatingparameters can provide an improved ride According to an embodiment ofthe invention, an apparatus is provided for adaptively controllingrelative motion between the wheels and the frame of a heavy duty truckhaving a frame suspended on wheels by a suspension system. An air springis disposed between the frame and at least one of the wheels having aprimary reservoir for holding air and a piston adapted to act upon theair in the reservoir to compress the air and thereby provide support forthe frame. An auxiliary reservoir for holding air that can be placed influid communication with the primary reservoir to increase an effectivevolume of air upon which the piston acts is mounted in proximity to theprimary reservoir. A control valve selectively places the auxiliaryreservoir in communication with the primary reservoir based on vehicleoperating parameters.

[0005] In a preferred embodiment, the auxiliary reservoir is adjacentthe primary reservoir and the control valve is disposed within a channelconnecting the primary reservoir to the auxiliary reservoir. The controlvalve includes a housing having ports disposed therein that connect theprimary reservoir to the auxiliary reservoir and a plunger forcontrolling the flow of air between the primary and auxiliaryreservoirs. The plunger is mounted within the housing and is moveablebetween a first position in which the primary and auxiliary reservoirsare not in fluid communication and a second position in which theprimary and auxiliary reservoirs are in fluid communication.

[0006] According to a feature of one embodiment of the invention, theplunger is in the first position under normal operating conditions andthe plunger moves to the second position when acted upon by relativelylarge impulse forces from the wheels of the vehicle. In an exemplaryembodiment, a controller is provided that monitors vehicle operatingconditions and that controls the control valve based on vehicleoperating conditions. The controller causes the control valve to placethe auxiliary reservoir in fluid communication with the primaryreservoir when relatively large impulse forces are experienced by thewheels.

[0007] In an embodiment, the controller determines a torque at thevehicle wheels and a road roughness indicator and causes the controlvalve to selectively connect or disconnect the primary reservoir to theauxiliary reservoir in response to the torque and road roughness. Thecontroller may monitor vehicle speed, engine torque, engine speed, andair spring pressure to determine the amount of torque at the vehiclewheels and the road roughness indicator.

[0008] These and other objects, advantages, and features of theinvention will be better understood from the accompanying detaileddescription of preferred embodiments of the invention when reviewed inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which:

[0010]FIG. 1 is a side plan view of a heavy duty truck suspension systemconstructed in accordance with an embodiment of the present invention;

[0011]FIG. 2 is a cross sectional view of an air spring of thesuspension system of FIG. 1; and

[0012]FIG. 3 is a flow chart outlining method steps that are used tocontrol the stiffness characteristics of the air spring of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013]FIG. 1 depicts a side plan view of a trailing arm torque reactivesuspension system 10 associated with a pair of rear wheel sets A and A′that drive a frame member 11 of a heavy duty truck. The suspensioncomponents associated with each wheel set are the same. The basiccomponents of the suspension system 10 should be familiar to one ofskill in the art and will only be outlined generally below.

[0014] The suspension system 10 includes a main support member 17 thatis pivotally connected to the frame 11 by a spring hanger bracket 19. Adrive axle 25 is attached to the main support member using a U bolt 29.A shock absorber 21 is connected between the main support member 17 andthe frame member 11 to damp the rise and fall of the frame of thevehicle with respect to the road. An air spring 23 (shown in more detailin FIG. 2) is bolted at its base to a base bracket 31 which is in turnconnected to the main support member 17. The top of the air spring 23 isattached to the adjacent side of the frame member 11 by means of abracket (not shown). The air spring provides cushioning for the loadborne by the vehicle frame as well as providing support to maintain theload at a desired height with respect to the wheels. The air spring alsoaffects the stiffness characteristics of the suspension by controllingthe level of impulse load that is transferred to the shock absorber 21.

[0015]FIG. 2 depicts a side view of the air spring 23. An air springprimary reservoir 33 is mounted to base bracket 31. An air spring piston34 protrudes into the primary reservoir 33 and is connected to thereservoir by a seal ring or rolling rubber sheath. The piston 34 acts tocompress the air in the primary reservoir. The pressure created withinthe reservoir by the protrusion of the piston into the reservoir createsan upward force on the vehicle frame 11 to support the frame. Whenforces are input into the system from the load on the vehicle frame orfrom the wheels, the air spring 23 cushions the ride. The stiffness ofthe air spring is inversely proportional to the volume of the reservoirupon which the piston 34 acts. The relationship between spring stiffnessK and the volume V of air in the reservoir is expressed in the equationK=αpA²/V, where α is a constant, p is the pressure within the reservoir,and A is the effective area of the air spring. The effective area of theair spring is approximately equal to the circular area determined by themeniscus of the rubber sheath surrounding the primary reservoir. As canbe seen from the preceding equation, the stiffness of the air spring 23can be reduced by increasing the volume of the reservoir. In FIG. 2, theauxiliary reservoir 35 is isolated from the primary reservoir 33 toprovide a lower effective volume and hence a stiffer spring.

[0016] The air spring 23 features an auxiliary reservoir 35 that can beselectively placed in fluid communication with the primary reservoir 33via control valve 37. The control valve 37 includes an actuatableplunger 42 housed within a valve housing 39. The valve housing 39 has aplurality of ports disposed about a periphery for connecting the primaryreservoir 33 with the auxiliary reservoir 35. When softer air springcharacteristics are desired, the plunger 42 is moved away from theprimary reservoir 33 to a position (shown in phantom) that openschannels of flow through the ports between the primary and auxiliaryreservoirs. In this manner, the volume of the reservoir upon which thepiston 34 acts is increased by the volume of the auxiliary reservoir 35to reduce the stiffness of the air spring.

[0017] In one embodiment, the control valve is operated in a passivemanner. Under normal operation conditions, the spring is set as shown inFIG. 2 to provide relatively high stiffness characteristics. When a highimpulse force is introduced to the air spring 23 from the wheels the airin the reservoir 33 is compressed to create a pressure that moves theplunger 42 away from the primary reservoir. When the plunger 42 is movedup to the position at which the auxiliary reservoir 35 is placed incommunication with the primary reservoir 33, the air spring stiffness isreduced to provide additional cushioning against the high impulse force.Once the force is removed, the plunger 42 returns to its normal positionunder the force of gravity or an assist mechanism such as a spring (notshown) and the system returns to its original stiffness characteristics.

[0018] In an exemplary embodiment, the control valve 37 is controlled bya microprocessor (not shown) that monitors various operating conditionsand actuates the plunger 42 to provide a softer air spring whendesirable. FIG. 3 is a flowchart that outlines steps that can be used tocontrol the plunger. The air spring begins in the softer mode, with theauxiliary reservoir in communication with the primary reservoir. Themicroprocessor monitors vehicle speed, engine torque, and engine speedin steps 54, 56, 58 and calculates a torque at the vehicle's wheels (Tw)in step 60. The air spring pressure is input to the microprocessor instep 62 to be used to calculate the road roughness (Pstd). In step 66,an aggregate constant k is determined by subtracting the road roughnessfrom a scaled version of the road roughness. If the aggregate constantis higher than a threshold, meaning that the road is relatively smoothor the suspension torque reaction is relatively low, the spring is setto the stiffer setting in steps 68 and 70. If k is lower than thethreshold, the spring remains in the softer mode as shown in step 69.Once the spring is set to the stiff setting, the algorithm delays for 30seconds and then recalculates the aggregate constant (step 72). If thenew value of the aggregate constant is still higher than the threshold,the spring remains stiff as shown in steps 74 and 76. If the constant islower than the threshold,the spring is set back to the softer setting instep 78.

[0019] As can be seen from the foregoing description, a heavy duty trucksuspension system having air springs whose stiffness characteristics canbe adjusted according to operating parameters can provide an improvedride. Although the present invention has been described with a degree ofparticularity, it is the intent that the invention include allmodifications and alterations from the disclosed design falling withinthe spirit or scope of the appended claims.

We claim:
 1. For a vehicle having a frame suspended, on wheels by asuspension system, an apparatus for adaptively controlling relativemotion between the wheels and the frame, the apparatus comprising: anair spring disposed between the frame and at least one of the wheels,wherein the air spring comprises a primary reservoir for holding air anda piston adapted to act upon the air in the reservoir to compress theair and thereby provide support for the frame; an auxiliary reservoirfor holding air wherein the auxiliary reservoir can be placed in fluidcommunication with the primary reservoir to increase an effective volumeof air upon which the piston acts; and a control valve for selectivelyplacing the auxiliary reservoir in communication with the primaryreservoir.
 2. The apparatus of claim 1 wherein the auxiliary reservoiris adjacent the primary reservoir and wherein the control valve isdisposed within a channel connecting the primary reservoir to theauxiliary reservoir.
 3. The apparatus of claim 1 wherein the controlvalve comprises a housing having ports disposed therein that connect theprimary reservoir to the auxiliary reservoir and a plunger forcontrolling the flow of air between the primary and auxiliaryreservoirs, wherein the plunger is mounted within the housing and ismoveable between a first position in which the primary and auxiliaryreservoirs are not in fluid communication and a second position in whichthe primary and auxiliary reservoirs are in fluid communication.
 4. Theapparatus of claim 3 wherein the plunger is in the first position undernormal operating conditions and wherein the plunger moves to the secondposition when acted upon by relatively large impulse forces from thewheels of the vehicle.
 5. The apparatus of claim 1 further comprising acontroller that monitors vehicle operating conditions and that controlsthe control valve based on vehicle operating conditions.
 6. Theapparatus of claim 5 wherein the controller causes the control valve toplace the auxiliary reservoir in fluid communication with the primaryreservoir when relatively large impulse forces are experienced by thewheels.
 7. The apparatus of claim 5 wherein the controller determines atorque at the vehicle wheels and a road roughness indicator and causesthe control valve to selectively connect or disconnect the primaryreservoir to the auxiliary reservoir in response to the torque and roadroughness.
 8. The apparatus of claim 7 wherein the controller monitorsat least one of vehicle speed, engine torque, engine speed, and airspring pressure to determine the amount of torque at the vehicle wheelsand the road roughness indicator.
 9. The apparatus of claim 1 comprisingan assist mechanism coupled to the control valve for causing the controlvalve to return a steady state position in which the auxiliary reservoiris not in fluid communication with the primary reservoir.
 10. For avehicle having a frame suspended on wheels by a suspension systemincluding an adaptive air spring comprising a primary reservoir, asecondary reservoir, and a control valve for selectively placing theprimary and auxiliary reservoirs in fluid communication to vary an airspring stiffness between a stiff mode resulting from the primary andauxiliary reservoirs not being placed in communication and a soft modein which the primary and auxiliary reservoirs are in communication, amethod for controlling the adaptive air spring comprising the steps of;monitoring vehicle operating parameters; causing the control valve toplace the primary and auxiliary reservoirs in communication in responseto the monitored operating parameters.
 11. The method of claim 10wherein the vehicle operating parameters comprise at least one of enginespeed, engine torque, vehicle speed, and air cylinder pressure.
 12. Themethod of claim 10 comprising the step of calculating a wheel torquethat is present at the wheels of the vehicle and a road roughnessindicator based on the monitored parameters.
 13. The method of claim 12comprising the step of calculating an aggregate constant by calculatinga difference between the wheel torque and the road roughness indicator.14. The method of claim 13 comprising the step of causing the controlvalve to place the primary and auxiliary reservoirs in communicationwhen the aggregate constant exceeds a predetermined threshold.
 15. Themethod of claim 13 comprising the step of causing the control valve totake the primary and auxiliary reservoirs out of communication when theaggregate constant is below a predetermined threshold.
 16. The method ofclaim 15 comprising the step of causing the control valve to maintainthe primary and auxiliary reservoirs out of communication for apredetermined delay period and then recalculating the aggregateconstant.
 17. For a vehicle having a frame suspended on wheels by asuspension system, an apparatus for adaptively controlling relativemotion between the wheels and the frame, the apparatus comprising: anair spring disposed between the frame and at least one of the wheels,wherein the air spring comprises a primary reservoir for holding air anda piston adapted to act upon the air in the reservoir to compress theair and thereby provide support for the frame; an auxiliary reservoirfor holding air wherein the auxiliary reservoir can be placed in fluidcommunication with the primary reservoir to increase an effective volumeof air upon which the piston acts; a road roughness monitor formonitoring the roughness of a road upon which the vehicle is traveling;and a control valve for selectively placing the auxiliary reservoir incommunication with the primary reservoir based on the monitored roadroughness.
 18. The apparatus of claim 17 wherein the road roughnessmonitor monitors road roughness by measuring pressure in the air spring.